Thermionic vacuum tube and circuits



June 10, 1941. R. H. VARIAN ETAL THERMIONIC VACUUM TUBE AND CIRCUITSFiled May 24, 1958 2 Sheets-Sheet 1 Eu: CTRON Lsnvss FILAMENT 1POTENTIAL or FILAfENTJ Aursnuarms Pan-aim. OF Gmo 2' ALTERNATING-POTENTIAL or GRID I5 INVENTOR. RUSSELL H MIR/AN F disarm ATTIORAV Y.

2 Sheets-Sheet 2 I INVENTOR. Russ/ELL H VAR/AN ARA/01.01 .ZZIEGERT IATTORNE 4 R. H. VARIAN ETAL THERMIONIC VACUUM TUBE AND CIRCUITS FiledMay 24, 1938 June 10, 1941.

Patented June 10, 1941 2,244,747 rnmmomc vacuum TUBE m cnwurrs RussellH. Varian and Arnold J. F. Siegert, Stanford University, Calf assignorsto The Board of Trustees of The Leland Stanford Junior University,Stanford University, Calit, a corporation of California Application May24, 1938, Serial No. 209,678

" 4 Claims.

.The present invention relates generally to means and methods forconverting direct or low frequency current into alternating current, andparticularly to alternating currents of frequenciens of 10 cycles ormore per second. Especially this invention has reference to electricalconverters, including oscillators, amplifiers, and detectors employingcontrol grids in connection with cathodes and anodes connected toresonant circuits.

The principal object of the present invention is to overcome alimitation inherent in the known types of thermionic three-electrodetubes, namely, the limitation dependent on active grid loss, and thus tocircumvent to a considerable degree the limitations of the known typesof tubes as to the frequencies attainable in their operation. Anotherobject is to provide a control grid arrangement in the class of tubesgenerally included in the classification three-electrode, includingtriodes, pentodes, and other conventional forms, which permits the gridimpedance to be, as may be desired, positive, negative or effectivelynearly infinite. Still another object is to make it feasible to makethree-electrode vacuum tubes for operation at high frequencies withoutextremely small spacings between the electrodes, thus also facilitatingthe manufacture of vacuum tubes for high frequency and large powerrating.

Other objects and advantages will become apparent from thespecification, taken in connection with the accompanying drawingswherein the invention is embodied in concrete form.

In the drawings,

Fig. 1 is a diagrammatic representation of a three-electrode tubecircuit as known in the prior art shown for use in explaining thepresent invention.

Fig. 2 is a representation of a three-electrode tube circuit embodyingthe present invention.

Fig. 3 is a graph explaining certain features of the invention.

Fig. 4 is a'detail showing a modification applicable to Fig. 2.

Fig. 5 shows an alternative form of grid structure.

Similar characters of reference are used in all of the above figures toindicate corresponding parts.

Referring now to Fig. l, which shows a conventional three-electrodecircuit, the phenomena of active grid loss which the present inventionovercomes will be explained. In Fig. 1 there is an electron emittingfilament I, a control grid 2,

and a plate 3 in an evacuated container 4. The

filament I is heated by a battery 5, the grid 2 is biased by a battery6, and the plate 3 is energized by a battery I. A resonant circuit 8comprising a condenser 9 and an inductance I0 impresses an alternatingdifference of potential on the grid 2 with respect to the cathode I. Aninductance II in series with the plate circuit is inductively coupled toinductance III for feedback control. A resistor I2 represents the loadto which the system delivers energy, and an inductance I3 connected to agenerator I l and inductively coupled to inductance It represents thesource of alternating current excitation for l the system. The system asshown is capable of operating as an oscillator, as an amplifier or as adetector depending on factors of design and adjustment. The generaltheory of operation is well known in the prior art and will in thefollowing be assumed without explanation except insofar as the effect ofactive grid loss is concerned.

In the operation of Fig. 1 at low frequencies the time required for anelectron to travel from the filament I to the plate 3 is small comparedwith the period, that is, the time corresponding to a cycle ofoperation. The grid 2' has its potential varied with respect to thefilament I potential at the frequency of the system, and the impedanceof the filament I to plate 3 space is varied in accordance with the grid2 potential. The result is that the grid acts as a valve or gatepermitting the electrons to pass through in varying numbers. Eachelectron permitted to pass the grid 2 jumps from the filament I to theplate 3 in a very short time so that any electron passing through grid 2is subjected to a substantially constant grid 2 potential duringthe timethe electron is passing from filament I to plate 3. Under theseconditions energy is not transferred between the grid 2 and theelectrons which pass through the grid. This statement should not beconfused with the fact that a positively charged grid carries current.To avoid possible confusion, however, the subject of grid loss will beexplained with reference to a grid which is negative with respect to thefilament at all times, and thus does not collect electrons from thesurrounding space.

It is well known in the art that so long as grid 2 remains at a constantpotential, it may control the number of electrons passing from electronemitter I to plate 3, but it cannot influence the energy with whichelectrons strike plate 3. This follows because whatever the potential ofgrid 2 may be, the electrons in passing the grid are merely passing apotential valley or bill as the case may be, and the energy lost by theelectrons in ascending the hill is all regained in going down the otherside. If the grid represents a potential valley, the same is true withthe signs reversed. The same is true also it the potential of the gridis changing slowly, and it is easily seen that it will remain true aslong as the grid does not change its potential appreciably while theelectron is in transit between filament I and plate 3.

It on the other hand the grid 2 does appreciably change its potentialwhile an electron is in transit between filament I and plate 3, theelectron may strike the plate with either increased or diminishedenergy, for if the height of the po-' tential hill, or the depth of thepotential valley, at grid 2 changes while the electron is in transit,the energy lost on the ascent side will in general not equal the energygained on the descent side. The matter of whether the electron gains orloses hill or valley at grid 2 depends on the phase or the change whenthe electron passed through the field 01' grid 2.

If a stream of electrons uniformly distributed in time crosses thecyclically varying potential hill or valley at grid 2 there will be asmany electrons gain energy as lose energy, and if the gain or loss issmall compared with total energy, the cyclic variations in the barrier,that is, the potential of grid 2, will not increase or decrease theaverage energy with which the electrons strike the plate 3. However, ina three-electrode tube the electron stream is not uniformly distributedin time, and it therefore becomes necessary to investigate the phaserelations existing between the maximum electron emissionand the gridpotentials to determine whether the electron stream on an average gainsenergy from, or loses energy to, the grid circuit. The greatest numberof electrons will leave the filament I when the grid 2 is most positive,and these electrons will gain energy from the grid circuit in travelingfrom the filament I to the grid 2, and since the grid 2 will be morenegative while the electrons complete their journey from the grid 2 tothe plate 3, these electrons will not lose all the energy they gained intraveling from the filament I to the grid 2 or may even acquire moreenergy from the grid in their flight to the plate. Hence the grid 2 willlose energy to the electron stream. This is known as active grid loss.

. In thetube as shown in Fig. 1, at high frequencies, the time requiredfor an electron to travel from the filament I to the plate 3 may becomecomparable with a period of oscillation of the system. In tubes ofordinary dimensions the transit time in the tube becomes comparable withthe period at frequencies of the order of cycles per second or less, thelarger the tube in general the lower the frequency where transit timebecomes appreciable. When the transit time of the electron travelingfrom filament I to plate 3 is an appreciable fraction of the oscillationperiod the potential of grid 2 with respect .to filament I changesmaterially during the time oi transit of the electron from the filamentI to plate 3, and the tube is thus subject to active grid loss.

The active grid loss is understood in the prior art. Some attempts toovercome this loss have been made, in particular attempts to reduce theenergy'as a result of the change in the potential transit time ofelectrons in the tube by reducing the spacing between the electrodes,but none of the attempts known prior to the present invention did morethan reduce the efl'ect by dimensional design.

In the present invention th factors which determine active grid loss arecontrolled in a definite way so the active grid loss may be eliminatedentirely or made negative so that the grid 2 can receive energy from theelectrons in their transit from the filament I to the plate 3. This isaccomplished by shifting the phase of electron departure from the grid 2with respect to the phase of electron entrance to the grid 2. This canbe done in at least two ways.

A general way of controlling active grid loss is accomplished in thearrangement shown in Fig. 2 in which all the elements shown in Fig. 1are present with the addition of a second grid or grid section I5, achoke I6, and capacitance elements I! and I8. In the operation of thesystem shown in Fig. 2, the oscillations are produced in exactly thesame way, in general, as in Fig. 1. from tuned circuit 8 to grid I5.Grid I5 is held at a direct current potential which is positive, withrespect to the cathode I, by connection to the plate battery 'I. Thedifierence in direct current potential between grids 2 and I5 isrequired in order to avoid excessive grid impedance. If it were not forthis, grids 2 and I5 would be connected together, an arrangementdesirable in some instances but not applicable in general.

The capacitance between grids 2 and I5 as especially provided by platesI1 and I3 is used to transmit alternating voltages from grid I5 to grid2. Since the coupling of grid 2 to other elements of the tube is mainlycapacitive, the alternating voltage ongrid 2 will be in approximatelythe same phase as that on grid I5. The choke I5 is used to avoid thebypassing of alternating current from grid 2 to filament I.

Grid section 2 acts as the control grid, as usual, and by varying thespace charge in front of the cathode regulates the departure ofelectrons from the filament I. The electrons in traveling from filamentI to plate 3 transfer energy to or from grid 2, as explained before. Ifthe electrons leaving filament I are, for example, approaching grid 2during the time when grid 2 is positive and becoming more negative theywould in an ordinary tube as shown in Fig. 1 recede from grid 2 during atime when grid 2 is more negative than when the electron is vapproaching the grid. But if the electron were delayed at grid 2 for asuitable fraction of a cycle, it could traverse the remainder of thedistance to plate 3 in a phase relation to the grid swing that wouldcause the electron receding from grid 2 to do a maximum amount of workon grid 2. v

()bviously it is inconvenient to stop and start electrons at grid 2, butthe same effect can be had by making grid 2 in effect occupy enoughspace between the filament I and the plate 3 to permit the electron toenter grid 2, remain in efiect on the grid for a suitable time and thenleave the grid and proceed toward the plate 3. This is accomplished asshown in Fig. 2. The electron enters grid 2 from the filament I, and

, proceeds from grid 2 to grid I5 in a time which This potential iscommunicated are approximately at the same alternating potentialalthough at diii'erent direct potentials. The direct difference ofpotential in the case under consideration makes no difference in thetheory because it is the variation in alternating potential of the gridthat is responsible for the active grid I loss. The effect of thedifference of direct potential between grids 2 and I is merely to imparta constant acceleration to the electrons between grid 2 and grid I5which is independent of the alternating potentials of the grids.

Suppose an electron leaves filament I at the instant when grid 2 is atthe least negative part of its cycle, and that it reaches grid 2 onequarter of a period later. The electron will have been acted upon by analternating component of potential in the region from maximum positivealternating value to zero alternating vaiue. Now suppose the electrontakes one-half period to reach grid I5. It will then leave grid I5 atthe time when the alternating component of potential is again at zeroand is starting toward its maximum positive value. If the tube isdesigned so that the electron requires either one-half or one-quartercycle to reach plate 3 from grid I5 it will be subjected to analternating grid po ential, in. its course from grid I5 to plate 3, ofthe same value as that to which it was subjected in traveling fromfilament I to 'grid 2. Under these conditions no energy will have beentransferred to or from the tuned circuit 8 by the group of electronsconsidered and there will be no active grid loss due to this group ofelectrons.

In the tube shown in Fig. 2 it is possible by correctly proportioningthe inter-electrode spaces to regulate the exchange of energy from theelectrons to the grids 2 and I5 so the electrons deliver energy to thegrid circuit 8 through grids 2 and I5. This in effect establishes anactive grid gain analogous to the active grid loss described before. Ifthe grids 2 and I5 gain energy every cycle the system can be maintainedin oscillation without resorting to feedback of energy from the platecircuit coupling coil II to the grid circuit inductance II). This, ofcourse, has advantages of reducing circuit equipment and the lossesthereof. arrangement coil II, inductance I3 and generator I4 areeliminated and energy may be taken from circuit 8. For operation withactive grid gain, either the interelectrode spacings are made so thatcomparatively little energy is taken from the grids 2 and I5 by theelectrons in their travel from filament I to grid 2, and more energy isgiven to the grids 2 and I5 in the travel of the electrons from grid I5to plate 3, or the energy lost by the grids to the electrons intraveling from filament I to grid 2 is allowed to be what it may, andsteps are taken to cause the grids to gain an abnormally large amount ofenergy from the electrons as they travel from grid I5 to plate 3.

Either of the alternatives mentioned may be" accomplished with theapparatus shown in Fig. 2. To accomplish the first alternative it isnecessary that the flight time between cathode I and grid 2 beapproximately -cycle so that the electrons when they reach grid 2 havenot on an average derived energy from the grid. In this case grid I5 mayhave the same alternating potential as grid 2, as will be the case ifcapacity I|-I8 is made large. To achieve the second alternative, grid I5is caused to have a larger A. C. potential than grid 2, but of the samephase.

In such an This may be accomplished by reducing the size of capacity II-I8, so that grid I! has a considerably higher A. C. potentialresulting in the to plate 3, as will further appear. In this way it ispossible to increase the energy gained by the grids from the electronsin their flight from grid I5 to plate 3, and at the same time to gainsome energy from the electrons while they are in the interspace betweengrid 2 and grid I5. Because of the increased energy acquired by the gridstructure from the electrons in the second alternative it is possible toplace grid2 much less than -cycle from cathode I, as is indicated in theflight times shown in Fig. 3, because the initial loss of energy by grid2 can be more than made up in the remainder of the electron flight time.

The operation of this second alternative arrangement may be explained,with reference to Fig. 3, by considering an electron startin fromfilament I at the time to when grid 2 is at the most positive part ofits cycle. The electron arrives at grid 2 at time t1 while grid 2 isstill positive with respect to filament I, and thus absorbs energy fromgrid 2. This is represented by the shaded area under the curverepresenting the alternating potential of grid 2. This area representsthe integrated product of force on the electron multiplied by thedistance it travels. Passing through grid 2, the electron travelsbetween grids 2 and I5 for nearly three-fourths of a period. Theelectron absorbs a small amount of energy from the grid structure due tothe difference of potential during the remainder of the quarter cycle inwhich it started from filameni: I i. e., between t1 and 252. This isshown by the shaded area extending partially between the grid 2 and gridI5 and between the potential curves above the axis of the curves. Duringthe next half cycle i. e., between t: and t3, the electron transfersenergy to the grids as shown by the shaded area between the gridpotential curves below the curve axis. The electron leaves grid l5 justas the potential of the grids 2 and I5 with respect to the filamentreverses and becomes positive again. In moving from grid I5 to plate 3during the interval between is and t4, the electron is attracted andretarded by the positive grid and thus energy is transferred from theelectron to the grid. The energy transferred is represented by theshaded area between the curve axis and the potential curve extendingbetween grid I5 and plate 3. The net transfer of energy between theelectron and the grids in this instance is from the electron to thegrids and is the maximum possible under the assigned conditions. Anyother electron leaving earlier or later in the cycle than the particularelectron discussed would transfer less energy to the grids, or wouldreceive energy from the grids. It should be kept in mind in consideringFig. 3 that the potential differences concerned are related to the"alternating grid potentials and that they should not be confused withthe unvarying voltages,

ment l at the same rate throughout successive cycles, the net totaltransfer of energy between the electrons and the grids would be zero.Actually, the electrons leave the filament at a greater is introducedinto the mechanism of energy transfer between grids and electrons.

In the foregoing description, the energy interchange between theelectron stream and the grid structure is illustrated on a curverepresenting the potentials of grids 2 and i and further, in thedrawings, the distance the electron travels is represented as the samefor equal successive intervals of time. To do this simplifies thedrawings. but it must be pointed out that this renders it qualitativeonly, since it is the integral of potential gradient with respect todistance which gives the energy interchange and not the integral of thepotential on the grids with respect to time. To make the curve quan- Ititatlvely correct it is only necessary to use a relation of gradienttopotential taking into account the effects of tube dimensions and thechanges of electron velocity in the tube.

In some arrangements the grids 2 and I5 can be connected together,forming in effect a thick grid. One such arrangement is shown in Fig. 4in which a thick grid is used in an oscillator utilizing the enclosedresonator type of circuit described in copending patent application ofW. W. Hansen, filed July 27, 1936, Ser. No. 92,787.

In Fig. 4 the emitter is a heated surface 2| instead oi a simplefilament. Surface 2| is heated by an element 22. Surface 3 is a portionof the wall of a resonator opposite the emitting face 2| andconstituting the plate of the circuit. Between the emitter 2| and plate3 there are the two grids 2 and i5 connected together by the conductors25. The grids 2 and I5 are excited by a loop 28 in the resonator field.A grid leak is provided and connected to the resonator wall so as to beat the potential of the emitter 2|. The two halves 23 and 2| of theresonator have capacitive flanges 28 and 29.;

The usual p ate battery I is connected to the plate 3 and emitter 2| asusual, and a choke 3| is shown in the positive battery I lead. A loop 32in the resonator field provides for removing energy from the system.

In the operation of the arrangement shown in Fig. 4, the ordinaryprinciples of grid control are employed, with the control of transittime between grids 2 and I5 as described before. The operation similarto an ordinary tuned plateuntuned grid oscillator. The field of theresonator constitutes'the tuned plate circuit and the loop 26 in theresonator field the untuned grid circuit.

The distance between the grids 2 and I5, as indicated, depends upon thefrequency of the system, and also on the voltage and othercharacteristics of the system.

An alternative form of grid arrangement is shown in Fig. 5. Itsoperation is substantially the same as explained for Fig. 2, but astructural ehangein grids is provided. In this arrangement grids 2 andii are connected together and a third grid 35 maintained at a positivepotential is'placed between them.

For quantitative consideration of the operation of the present inventionthe mathematical analysis following is applicable. This analysis hasreference to Fig. 2 regarding which, for purposes of computation,special assumptions are made and special notations are adopted. Theseare: The electrons are assumed to start from aplane, designated as 0,between filament l and grid 2 and parallel to grids 2 and I5 and plate 3all of which are assumed to be sections of infinite parallel planes. Theplane II is, of course. recognized as a virtual cathode, a conventionalready in common use; The electrons are assumed to move from thevirtual cathode plane 0 through grids 2 and I5 and to stop at plate 3without the possibility of any secondary emission. It is assumed alsothat grids 2 and I! do not collect any of the electrons but that theyhave only pure valve action on the electrons in transit. A furtherassumption regarding plate 3 is that the voltage on it is constant. Thiscondition can be attained in effect by putting a screen around plate 3at the plate potential. The screen is placed at the distance from gridi5 assumed for plate 3 and plate 3 is put farther away. In thisarrangement the screen grid becomes a virtual plate. Inasmuch as thevirtual plate effect of a screen grid is well known in the art and itsintroduction here is solely for convenience in assuming a constant platevoltage, it has not been shown in the drawings, nor is the nomenclaturecomplicated by its addition. Reference to plate 3 will in all instancesimply the conditions stated for constant plate voltage. For conveniencein the mathematical nomenclature the plane of the virtual cathode issaid to be at 0, the planes of grids 2 and I5 are said to be at l and 2respectively, and the plate 3 at 3. The distance between the virtualcathode 0 and the grid 2 is a, that between grids 2 and I5 is b, andbetween grids I5 and plate 3 is c. The distance to any plane in the tubefrom the virtual cathode 0 is x. In the analysis the power lost orgained by the grids 2 and I5 is calculated disregarding the eifect ofvelocity grouping of electrons.

In the spaces a, b, and c the electric fields are given by:

Field in'space a: EE+E1 cos at Field in space b: E2

Field in space c: E2+E' cos mt where w is Zr, and f is the frequency ofoperation of the tube and E stands for a constant field, and thesubscripts a, b, and c" designate the spaces in which the respectiveconstants fields E" exist. Also, E stands for the peak A. C. field, andthe subscripts a and "c" designate the spaces in which the A. C. fieldsexist, just as in the case of the D. C. potentials.

It is assumed here that: (1) all the ES are constants and that In thisequation the integral subscripts and superscripts indicate theintegration from the re- 'and (3) tiW=eJI EL cos wide-i1]: E; cos aids:

The first part W is the energy obtained from the unidirectional plate 3potential. The second part AW is the energy gained from the alternatingcomponents of the interspace fields due to the efiects of the controlgrids. It will be noticed here that there is no alternating component offield in space b between grids 2 and I5, inasmuch as condenser plates l1and I8 in Fig. 2 provide a low impedance path for the alternating gridcurrents.

To calculate the power loss of the control grids, AW is multiplied bythe number of electrons leaving the virtual cathode 0, during the timebetween t=to and t=to+dtu, that is, i+Ai cos wto and to integrate overone period. This is accurate providing the operation of the tube is inthe linear part of its characteristic in whichi and A1 are constants.

The power loss is then I= eq y. and o=wto- From elementaryconsiderations,

Equation 3 as now written is substituted into Equation 4 as follows:

1 21 l I 5 AE== L {6L E,

{i-l-Ai cos 6 bit. Since the unidirectional component i of the currentdoes not contribute to the power loss. Further, since 2 L sin 5 cos 6 d5=O none of the terms containing sin 60 contribute. Therefore, using 2 icos 6 116 g EL cos w(t t )dx+f E; cos ecu-t ws} This equation candirectly evaluated quantitatively for small values of t-to, arestriction not ordinarily desired in practice, but for more generalcases quantitative evaluation is better accomplished by an indirectmethod. For this the mean value theorem is applicable.

The mean value theorem states that for any function #0:) continuous inthe interval a z b with a :r b. For approximate solution a: does nothave to be evaluated.

Therefore, v

where ta denotes a moment of time which althoughdefinite is notevaluated more precisely than to say that at time t. an electron thatstarted from the vertical cathode 0 at time to is in the region abetween filament l and grid 2. In this equation the locations ofspecified electrons in the regions a and c are not ascertained moreclosely than the limits of the regions themselves to which the instantsof time ta and to refer because of the approximation involved in the useof the mean value theorem for obtaining a solution of the problem.Consequently the (9) {cos Mi -t cos Mi -t9} as the active grid powerloss. 7

A complete understanding and interpretation of this equation indicatesthe dimensional characteristics of the embodiments of the presentinvention shown in Figs. 2, 4 and 5 which have already been explained ina qualitative way, and in addition indicates how the ordinarythreeelectrode tube shown in Fig. 1 can be modified for the partial orcomplete elimination of active grid loss. It also indicates how the tubeof Fig. 1 can be made to oscillate without the grid 2.

In the ordinary three-electrode vacuum tube as shown in Fig. 1 the time(to-t0) for an electron to get into the region between grid 2 and plate3 is a small portion of a period of oscillation. That is (tn-t0) w 1 and(to-t0) w 1, and accordingly the cosines of these angles are both closeto 1. In this region of the cosines, cos :c e -:c /2. Using thisapproximation for the cosines, Equation 9 isapproximated by whichsimplifies to (11 A g gut-o -c.-o 1} This equation indicates that withinthe ordinary ranges of frequencies encountered in triode practice theactive grid loss increases as the square of the frequency. This equationrepresents the operation of triodes inasmuch as (ts-4o) is alwaysgreater than (ta-t0) as long as (to-t0) is one-half period or less ashas been the practice in the prior art.

Returning now to Equation 9 it may be seen that if, for instance,wan-to) and Ate-to) are both in the region between zero and 1r thequantity in the brackets will always be positive because the differencebetween the cosines will always be positive since (tr-to) is necessarilyless than (tto). But if (ta-t) and (to-to) are increased by increasingthe distances inside the tube until Mic-to) is increased to a valuebetween 1r and 21-, the cosine of the angle will begin to increaseagain. 7

This can be accomplished by increasing the spacings between the virtualcathode 0 and the grid 2, and between the grid 2 and the plate 3suiiiciently so that w(tc't0) is of the order of 21r. This is in directcontradiction to the practice of the prior art in which triodes for highfrequency have been made with interelectrode spacings assmall aspossible. By suitably spacing the electrodes of the tube with referenceto the frequency and voltage of the system the grid loss thus can bemade either positive; negative, or zero. If it is made negative, thegrid 21 will gain energy from the electrons, accomplishing thereby theconversion of direct to alternating current directly within the gridcircuit.

Further examination of Equation 9 indicates that if the grid 2 isremoved entirely only one cosine term will remain. This is cos Mia-to)and the plate then has the mathematical meaning formerly given to grid2. As is evident from Equation 9 it is only necessary to make this termnegative to accomplish active grid gain instead of active grid loss.Accordingly it is only necessary to restrict the term ode-to) to theregion between 1/2 and 37/2 in order that its cosine will be negative.Under these conditions the electrons leaving the virtual cathode 0 att=tn will leave there in varying numbers per unit of time as usual inresponse to the alternating component of voltage between the cathode andanode. The greater number per unit time will leave the cathode when thealternating component is a maximum. These electrons will arrive :at aplane in the region a through which the electrons pass at time is about/2 period after they leave the virtual cathode 0. It should perhaps benoted again that this'plan'e is not the plane of the physical plate, forin using the mean value theorem we merely showed that such a definiteplane existed, and did not. define its position more precisely than tosay that it lay somewhere in the region a. Under these conditions cosMia-to) has a minimum value and is negative, and hence the electronspassing across the space between cathode and plate will deliver energyto the resonant circuit connected to the plate. This condition can besatisfied if the plate 00- comparatively large. This construction hassome disadvantages which are overcome by the construction of Figs. 2, 4,and 5.

In Figs. 2, 4, and 5, til-to can be made as small as may be desired,that is, grid 2 may be placed as near the filament l as is convenient.Then by making the time of flight between grids 2 and i5 suiiicient thespacing between grid I5 and plate 3 can be made either large or small.If, for example, grid 2 is very near filament I, and il-t0 is verybrief, cos w(tat0) is approximately 1. If now the space between grids 2and I5 is made suflicient so that ode-to) is approximately 2x, thequantity cos w(tat0)COS w(te+to) will approximate 1l=0 and there will belittle or no active grid loss. If, as a second example, the spacebetween filament i and grid 2 is made considerable so that w(tat0)approximates 3r,

' and then the spaces between grids 2 and I5 and (Ate-to) cos ode-to)will approximate cupies some definite position beyond the plane throughwhich electrons pass at the time ta,

and hence the diode may become self -oscillatory.

For this type of oscillator the arrangement shown in Fig. 1, with grid 2and resonant circuit 8 both omitted and with a plate circuit condenser Hfor use with inductance ll added, is satisfactory.

In the preceding discussion of Equation 9 as applied to Fig. 1 it wasshown that the active grid loss can be reduced to zero or made negativeby making the space from the cathode to the grid This means that insteadof active grid loss there is an active grid gain, and a consequence ofthis condition is self-oscillation of the system.

In general, for maximum grid gain of power, that is, for maximumtransfer of energy from the electrons to grids 2 and IS, theinterelectrode spacings and the voltages are adjusted so that w(ta--to)a 1|- or any odd multiples of r, and (d(tct0) g 21r or any evenmultiples of 1. Emphasis is again placed on the fact that t. and to arenot the precise instants of time when the electrons reach the planes ofgrid 2 and plate 3 respectively, but are times prior to the instants oftime when the electrons reach these two electrodes.

As many changes could be made in'the above construction and manyapparently widely difl'erent embodiments of this invention could be madewithout departing from the scope thereof, it is intended that all mattercontained in the above description or shown in the accompanying drawingsshall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. In an electrical converter, a closed hollow conducting bodycomprising capacity coupled body half sections, means for establishing adirect current potential between opposing inner surfaces of said bodyhalf sections, means for causing thermionic emission from one of saidopposing surfaces, and a grid structure interposed between said surfacesand comprising a pair of mutually spaced grid sections, a conductingloop within said body connected to said grid sections for exciting thelatter, the flow of electrons within said body acting to build up asuitable transient and establish an oscillating electromagnetic fieldwithin said body for exciting said loop and effecting space chargecontrol of the electrons, the flight time of the electrons between saidgrid sections being substantially one-half cycle of oscillation of thefield within said body.

2. A thermionic tube constructed for operating at high frequencies sothat the grid changes in potential during the transient time of theelectrons comprising, a cathode, a control grid acting directly on thecathode and operating on the principle of space charge control of thecurrent leaving the cathode, said control grid having spaced, mutuallycoupled grid sections, and a plate, the spacing of said grid sectionsproviding an electron flight time therebetween that is substantiallyone-half cycle of the operating frequency of the tube, oscillatorycircuit means including capacity elements connected with said gridsections for establishing desired alternating potentials thereon whilemaintaining th interspace between said grid sections substantially freeof alternating electric fields, and means for maintaining said gridsections at differing direct current potentials and for maintaining saidplate at a higher direct current potential than either of said gridsections.

3. A thermionic tube constructed for operating at high frequencies sothat the grid changes in potential during the transient time of theelectrons, comprising, a cathode, a control grid acting directly on thecathode and operating on the principle of space charge control of thecurrent leaving the cathode, said control grid having spaced gridsections, and an anode, the spacing of said grid sections providing anelectron flight time therebetween that is substantially one-half cycleof the operating frequency of the tube, means for applying alternatingcontrol potentials to said grid sections, means for applying a positivepotential to said anode with respect to said cathode and grid, said gridsections enclosing a region of space that is substantially free ofalternating electric fields and is shielded from the fields existingbetween the control grid and the cathode and anode, the spacing of saidcontrol grid sections from one another and the spacing of the first gridsection from the cathode being such that the electrons consumesubstantially three-fourths of a cycle in transversing the space fromthe cathode-to the second grid section, the majority of the electronsdoing work on the varying control grid field during the operation of thetube.

4. An electrical converter comprising, a vacuum electric tube having acathode, a grid structure acting directly on said cathode, and an anodeheld at a positive potential with respect to said cathode and gridstructure, said grid structure comprising, a pair of mutually spaced endgrid sections electrically interconnected so as to provide a spacetherebetween substantially free of alternating electromagnetic fieldsand a central grid section maintained at a positive direct currentpotential, means for exciting said grid structure to efiect space chargecontrol of electrons as they leave said cathode thereby alternatelyvarying the density of the electron stream leaving the cathode, thespacing of said grid structure end sections being 'such that theelectrons spend substantially one-half cycle of the control frequencyduring their passage within said grid structure whereby a phase delay ofsubstantially one-half cycle is produced in the entry of the electronsinto the field existing between the control grid structure and the anodeso that the electrons are caused to impart a maximum amount of energy tothe grid circuit.

RUSSELL H. VARIAN. ARNOLD J. F. SIEGERT.

